1. Field of the Invention
The present invention relates generally to test trees for testing and producing offshore wells, and particularly to a modification of such test trees to allow the test tree to cut larger diameters of coiled tubing when the test tree is closed in an emergency situation.
2. Description of the Prior Art
In the production testing of offshore wells, it is desirable to be able to quickly disconnect the production test string from the well in the event of an emergency such as adverse weather conditions. In making the quick disconnect, provision must be made for shutting in the well. This is commonly accomplished with a valve apparatus referred to as a subsea test tree. A subsea test tree is a type of safety valve.
A typical example of a prior art subsea test tree is seen in U.S. Pat. No. 4,494,609 to Schwendemann. Quite often when the production test string is in place in a well, it will be necessary to run other tools down through the production test string and down through the subsea test tree. These other tools are typically run on a wireline or on coiled tubing.
It is common practice with subsea test trees such as that of Schwendemann, when an extreme emergency situation arises, to close the ball valve member of the test tree while the wireline or coiled tubing still extends through the test tree, thus severing the wireline or coiled tubing by the shearing action of the ball valve member against its seat.
As procedures utilizing coiled tubing have evolved, the industry is moving toward use of larger and larger diameters of coiled tubing. This presents an increased difficulty in severing the coiled tubing in emergency situations with a subsea test tree.
Various approaches have been suggested to improve upon the capability of a subsea test tree for cutting these larger strings of coiled tubing.
One approach is that shown in U.S. Pat. No. 4,009,753 of McGill et al. wherein a slot is cut in the lower portion of the spherical ball valve member so that when the ball valve moves to its closed position with a string of coiled tubing still in place, the lower portion of the coiled tubing string will be received in the slot of the ball valve and thus will not be placed in double shear type bending as the ball valve closes. This is best illustrated in FIG. 6 of McGill et al.
Another approach to the problem is seen in U.S. Pat. No. 4,160,478 to Calhoun et al. The Calhoun et al. device does not use a conventional spherical ball valve member, but instead uses a combined cutter/valve operator member mounted eccentrically within the housing and associated with a spherical seat surface which is also developed on an eccentrically positioned center.